Photolithography in the production of integrated circuits is predominantly carried out by optical means. Photolithography is in many ways the key to microelectronic technology. It is involved repeatedly in the processing of any device, at least once for each layer of the finished structure. An important requirement of the lithographic process is that each pattern be positioned accurately with respect to the layers under it. One technique is to hold the mask (which serves as a template) just off the surface and to visually align the mask with the existing patterns in the wafer. After perfect alignment is achieved, the mask is pressed into contact with the wafer. The mask is then flooded with ultraviolet radiation to expose the photoresist. The space between the wafer and the mask is often evacuated to achieve intimate contact.
Problems in the masking process arise from the need to print very small features with no defects in the pattern. If the mask were to be positioned very far from the surface, diffraction of the ultraviolet radiation passing through the mask would cause the smaller features to blur together. On the other hand, small particles on the wafer or the mask are abraded into the mask when it is pressed against the wafer. Hence the masks can be used for only a few exposures before the abrasion causes defects to accumulate to an intolerable level.
A more recent trend has been toward using the technique known as projection alignment, in which the image of the mask is projected onto the wafer through an optical system. In this case, mask life is virtually unlimited. Projection alignment is now the predominant method used in photolithographic production of semiconductors. However, the image resolution of projection alignment lithographic systems is approaching the physical limits imposed by practical constraints on numerical aperture and wavelength. Although further improvements in lithographic technology are anticipated, dramatic improvements in inherent lens resolution are not. In order to continue the reduction of minimum feature size achievable by optical techniques, it is necessary to alter some other aspect of the lithographic process for further improvements. The photoresist exposure and development process is one area in which further improvements are possible.
A photoresist is a radiation-sensitive coating that is applied to a substrate, exposed to an image, and developed by a process which selectively removes (or leaves) the resist material that was exposed. For example, a negative photoresist may cross-link and polymerize upon exposure to ultraviolet or other types of radiation. Thus, exposure of the negative photoresist through a mask, followed by a development step (which consists of washing away the non-cross-linked material using selective solvents) results in the removal of the photoresist wherever the mask was opaque.
High contrast is necessary to produce on photoresists the image patterns used in the integrated circuit art. The minimum required contrast of illumination is referred to as the "contrast threshold" of the resist. Depending on substrate properties, the required pattern thickness and resist edge profiles, a conventional positive photoresist has a contrast threshold between about 85% and 90%. Currently, most production is done at 90% contrast, or more. If the contrast threshold of the resist is reduced, the resolution obtainable with a given optical system is improved due to the fact that image contrast is a decreasing function of the spatial frequencies present in the image.
Projection lithography generally uses an aerial image of a mask to expose the photoresist. But as the contrast is reduced, discrimination of a darker area from a lighter area becomes increasingly difficult. For an aerial image of low contrast, even those parts of the image that correspond to the dark regions of the masks have significant intensities. Hence, exposure of the photoresist using a mask of insufficient contrast, causes even dark areas to be exposed to a significant extent. On development, a blurred, poorly resolved photoresist is obtained.
A similar problem occurs in the optical projection of the mask image. The projected radiation has a tendency to scatter due to diffraction around the edge of a light/dark area in the image. The farther in proximity printing, the image is from the resist, the greater the amount of radiation scatter under intended dark areas of the resist.
To improve contrast, a contrast enhancement layer is used in conjunction with an underlying positive or negative photoresist layer.
European patent application No. 110,165 filed on Oct. 29, 1983 in the name of Griffing and West discloses a contrast-enhancing layer used in conjunction with a photoresist layer. The contrast-enhancing layer consists of a photobleachable material in a resinous binder. The contrast-enhancing layer is applied to the photoresist layer and forms a second, in situ, mask upon exposure to light. Light travels through the mask and acts on a photobleachable dye contained in the contrast-enhancing layer. Areas of the contrast-enhancing layer corresponding to the mask pattern become transparent, and allow the photoresist layer located below the contrast-enhancing layer to be selectively exposed to light according to the pattern of the mask.
Because a certain amount of light is required to render the photobleachable dye transparent, the photoresist layer is exposed only after the photobleachable dye has been made transparent. Dark areas of the mask allow less light transmission and, therefore, take longer to bleach the photobleachable dye of the contrast-enhancing layer. Correspondingly, less light is allowed through the bleached areas of the contrast-enhancing layer to expose the photoresist layer in the dark areas. Hence, the contrast-enhancing layer effectively increases contrast and the net result is a resolution improvement over using the positive resist alone. After the photoresist and contrast-enhancing layers have been exposed, the contrast-enhancing layer is stripped from the exposed photoresist with an organic solvent before development of the exposed photoresist.
Although the contrast-enhancing layer of Griffing and West is capable of increasing contrast and resolution, use of the layer in photolithography requires several extra steps which increase process time and reduce yield and efficiency.
It is an object of the present invention to provide a contrast-enhancing layer which can be used in a standard aqueous photoresist development step without requiring a separate contrast-enhancing layer stripping step prior to photoresist development.
Another object is to provide a CEL that does not intermix with the photoresist it coats.
Another object is to provide a CEL that shows a large decrease in absorbance at 300-450 nm when exposed to those wavelengths.
Another object is to provide a more improved CEL layer capable of further enhancing contrast and/or requiring shorter exposure times.